While I understand that reasoning, I have always thought that even in transformation there will be always be those "Truer" forms, and the knowledgeable would know how to discern them. Yes, this does still cause for the copycats, and the professional criminals to slip through the cracks, but really thats not much different from this reality.
It also would be completely reasonable that with the abundance of such magics that there would be the magic to negate or prevent a transformation on such a place as a witness stand...or a building in the case of preventing a "long range TF Hit".
This kinda leads me to my own reasonings to who would reside. To me it would be a group much like the courts are not who are skilled in the arts as much as the system, sort of like a circle of mages. This of course makes appointments of justices far more based on test than opinion, but it does to me at least make sense.
@Geary: That would be interesting, although...wouldn't that cause major uncomfertabilities without changing much Bias?